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Huge Ocean Confirmed Underneath Solar System's Largest Moon
sciencehabit writes The solar system's largest moon, Ganymede, in orbit around Jupiter, harbors an underground ocean containing more water than all the oceans on Earth, according to new observations by the Hubble Space Telescope. Ganymede now joins Jupiter's Europa and two moons of Saturn, Titan and Enceladus, as moons with subsurface oceans—and good places to look for life. Ceres, the largest object in the asteroid belt, may also have a subsurface ocean. The Hubble study suggests that the ocean can be no deeper than 330 kilometers below the surface.
Except it is not a solar radiation you need protecting against (Sun is very far), but Jupiter radiation. Unfortunately despite magnetosphere, Ganymede gets around 8 rem of radiation per day (http://en.wikipedia.org/wiki/Colonization_of_the_outer_Solar_System#Ganymede), which is bit too much for life as we know it. Fortunately, it is not going to be an issue 300km below the surface - but at that depth, you don't need magnetosphere anyway.
I think that biggest problem for life there would be availability of energy. 300km of crust is probably shielding external energy too well, so internal heat would be probably only viable source of that. Might be not enough to sustain life (or even more, to produce it randomly)
On the other hand, heat built up by tidal forcing of Jupiter will also not be able to escape easily, through such a thick crust. Remember that we have many deep-ocean creatures on earth, where there is no light, and the water is so cold that it is barely liquid. (zero to three degrees Celsius).
Actually, it has been estimated there are so many deep-ocean species, that bioluminescence might be the most common method of communication on earth.